Method for cooling a furnace, and furnace provided with a cooling device

ABSTRACT

A furnace includes a core tube that has an elongate boundary wall and is configured to accommodate wafers for processing the wafers in a treatment atmosphere. The furnace includes a cooling chamber defined between the elongate boundary wall and an outer casing of the furnace, wherein the outer casing includes a heating element and has first lateral, circumferentially spaced openings in proximity of a first end of the outer casing and second lateral, circumferentially spaced openings in proximity of a second end of the outer casing. Cooling gas is supplied through one of the first and second lateral, circumferentially spaced openings to a region of one end of the cooling chamber and provides for a cooling atmosphere. The cooling gas is guided along the cooling chamber with a uniform distribution of flow and discharged through one of the first and second lateral, circumferentially spaced openings from a region of an opposite end of the cooling chamber. A direction of flow of the cooling gas is periodically reversed during cooling, wherein the cooling gas is guided along an essentially closed circuit so that the cooling gas is preserved and so that the cooling atmosphere is separated from the treatment atmosphere. A liquid-gas heat exchanger cools the cooling gas.

The present invention relates to a method for cooling a furnace and afurnace.

BACKGROUND OF THE INVENTION

A method of this kind is known from American U.S. Pat. No. 4,925,388,which describes a furnace in which, for cooling purposes, a gas isguided through the furnace either from above or from below. Thedirection of flow of the gas alternates, and this alternating movementis controlled by a number of valves. The gas moves in the space betweenthe heating element and the core tube and in the space between theheating element and the insulation arranged at a distance therefrom. Theair or other gas which is used as cooling medium is discharged via afan.

It has been found that the discharge of such gases into a dischargesystem of a relatively large installation is accompanied by majordrawbacks, since by definition these gases are hot and consequently candamage a discharge system of this kind, in particular if it is composedof plastic components. Moreover, harmful substances may be present inthe gases.

The object of the present invention is to avoid this drawback and toprovide a method which, on the one hand, allows rapid and uniformcooling and, on the other hand, does not subject the gas-dischargesystem to further loading.

An aspect of the invention involves a method for cooling a furnacehaving a core tube that has an elongate boundary wall and is configuredto accommodate wafers for processing the wafers in a treatmentatmosphere. The furnace includes a cooling chamber defined between theelongate boundary wall and an outer casing of the furnace, wherein theouter casing includes a heating element and has first lateral,circumferentially spaced openings in proximity of a first end of theouter casing and second lateral, circumferentially spaced openings inproximity of a second end of the outer casing. Cooling gas is suppliedthrough one of the first and second lateral, circumferentially spacedopenings to a region of one end of the cooling chamber and provides fora cooling atmosphere. The cooling gas is guided along the coolingchamber with a uniform distribution of flow and discharged through oneof the first and second lateral, circumferentially spaced openings froma region of an opposite end of the cooling chamber. A direction of flowof the cooling gas is periodically reversed during cooling, wherein thecooling gas is guided along an essentially closed circuit so that thecooling gas is preserved and so that the cooling atmosphere is separatedfrom the treatment atmosphere. A liquid-gas heat exchanger cools thecooling gas.

SUMMARY OF THE INVENTION

In principle, it would be possible to discharge the gas solely via theheat exchanger and to guide it into the gas-discharge system withoutusing a closed circuit. However, the drawback of this is that it isconstantly necessary to introduce additional gas and, in the case ofair, this will generally emanate from the clean room, i.e., it will havebeen purified extensively. As a result, a particularly largeinstallation for cleaning air is required, with all the associatedcosts. Moreover, it is undesirable to periodically withdraw air from orintroduce air into a clean room.

If, in accordance with the invention, the gas is guided in a closedcircuit with a heat exchanger, it is not necessary to constantly supplyfresh gas, such as air. Moreover, it is possible as a result to make useof a gas having more specific properties, such as nitrogen. This isbecause basically there is no loss of gas, with the result that theextra costs of nitrogen or some other gas can be justified. The methodaccording to the invention makes it possible to treat a greater numberof wafers per hour, and the thermal loading to which each wafer issubjected will be reduced. Moreover, the treatment of the various wafersis more uniform.

It has been found that alternating the direction of flow of the coolinggas at a certain frequency enables the temperature gradient across thefurnace to be limited as far as possible. It will be understood that thehottest part will now be situated in the centre of the furnace.Moreover, it has been found that the cooling rate can be increasedconsiderably by comparison with designs in accordance with the priorart.

According to an advantageous embodiment of the invention, the reversalfrequency of the flow of gas lies between 2 and 600 seconds, and moreparticularly between 5 and 60 seconds. Advantageously, the reversalfrequency lies between 10 and 20 seconds. The above-described designmakes it possible to achieve a cooling rate in the region of the wafersof approximately 50° C. per minute.

Another aspect of the invention involves a furnace assembly, comprisinga furnace which is provided with a cooling device for using gas to coolthe boundary wall of said furnace, a cooling chamber for gas beingpresent in the region of said boundary wall, which chamber opens out onboth sides into a line which is provided with valves, which valvescomprise diverter valves, in order to move the gas alternately in onedirection along said boundary wall and then in the opposite directionalong said boundary wall, in which said diverter valves are connected toa closed circuit in which a liquid-gas heat exchanger is arranged.

According to an advantageous embodiment of the invention, thesupply/discharge of gas is arranged in the outer casing of the furnaceassembly, i.e. is effected via the wall and not via the end closures,such as the base or cover. This allows a more uniform distribution offlow to be achieved. This is because the movement of the gas in the gapbetween heating element and core tube is particularly difficult tocontrol. However, this control is important if sufficient uniformity isto be ensured. This distribution of flow can be promoted further byintroducing and discharging the gas via a number of openings which arearranged in a ring or other regular curve along the circumference of thefurnace wall. These openings form a restriction to the gas flow and infact locally determine the metering of the gas flow. By distributingthese openings regularly along the wall, it is possible to ensure aneven distribution of the gas flow, and hence an even dissipation ofheat.

BRIEF DESCRIPTION OF THE DRAWINGS

It is possible to design each diverter valve as a single or doublevalve, provided that the natural convection is eliminated.

The invention will be explained in more detail below with reference toan exemplary embodiment depicted in the drawings, in which:

FIG. 1 diagrammatically illustrates the principle of the method anddevice according to the invention;

FIG. 2 shows temperature curves of a number of wafers in a conventionalfurnace during cooling; and

FIG. 3 shows temperature curves at various levels in a furnace accordingto the invention during rapid cooling.

DETAILED DESCRIPTION OF THE INVENTION

The treatment furnace according to the invention is denoted overall by1, and is disposed in a treatment chamber which is otherwise depicteddiagrammatically. This furnace comprises an outer casing 7 made, forexample, of insulating material, against which a heating element 9bears. A core tube 5 made, for example, of quartz material or siliconcarbide is arranged at a distance from this diagrammatically depictedelement 9, in which tube it is possible to accommodate a boat 20provided with wafers 21-25. 18 denotes the top cover of the core tube.Wafer rack 20 is supported by cover 15, which can be moved up and downin a diagrammatically depicted manner in order to remove the boat 20from the core tube 5. Between heating element 9 and core tube 5, thereis arranged a gap 6, through which cooling gas, such as nitrogen, can bemoved. Two series of approximately sixteen openings 16 and 17 each arearranged in outer casing 7. Lines 3 and 8, which are respectivelyconnected to valves 12, 13 and 10, 11, are connected to these openings.These valves are in turn connected to a gas-liquid heat exchanger 4 anda fan 2.

The structure described above operates as follows. During cooling oftreated wafers, in a first operating condition cool gas emanating fromline 8 is introduced through openings 17 into the gap 6 between coretube 5 and heating element 9. This gas moves downwards in the directionof the illustrated arrow, and leaves the furnace in a heated state at16, after which it is guided via pipe 3 and valve 12 to heat exchanger4. After having been cooled, it is guided through fan 2 towards valve11, and is then fed back into line 8 and then to the treatment furnace.

After a certain time, such as typically 20 seconds, the direction offlow is reversed by switching over the valves, i.e. closing the valves11 and 12 and opening the valves 10 and 13. This means that cooling gasmoves from line 3 and openings 16 towards openings 17 and line 8. Fromline 8, the gas passes through valve 10 towards heat exchanger 4 and,via fan 2 and valve 13, back into line 3.

If the valves 10, 11, 12 and 13 are closed, the design in accordancewith the prior art would be the result. FIG. 2 shows a typical coolingrate for the wafers 21-25 in such a case.

The exit gas will be at a higher or lower temperature depending on thetemperature of the furnace. A value of the exit gas (air) of 300-400° C.is mentioned by way of example. If this gas is discharged via aconventional outlet system, this results in two problems. Firstly, it ispossible for low levels of pollutants, such as asbestos-like particles,or leakage gas to be present. Secondly, the increase in temperaturefrequently subjects the discharge system to impermissible loading. Thisapplies in particular if the discharge system comprises components whichare unable to withstand such high temperatures, such as plasticcomponents. In such a case, a discharge device of this kind is designedto carry out cooling, with the result that extra cold air is used. Thismeans that the discharge system has to be enlarged further in order tohave sufficient capacity for such rapid cooling of a furnace.

In order to avoid these problems, it is now proposed to use the heatexchanger 4, which is connected to the liquid-cooling system of theproduction installation. In general, the heat which is gained as aresult can be put to good use. In addition, an expensive gas can be usedin the cooling circuit, since there is no consumption of this gas.

By arranging the openings 16 and 17 in series in the wall andintroducing the cooling gas or air through these openings, it ispossible to obtain an optimum distribution of flow. It is assumed herethat the diameter of the openings 16, 17 determines the volume whichflows through, i.e. the flow resistance in gap 6 is much lower than theflow resistance through these openings 16, 17. In designs in accordancewith the prior art, the gas is introduced simply from above or below,and consequently it is impossible to ensure correct distribution of thecooling medium throughout gap 6, with the result that the temperaturedistribution is no longer uniform.

FIG. 3 shows the cooling rate of the same wafers when the gas flow isreversed every 20 seconds in accordance with the invention.

A comparison between FIGS. 2 and 3 will firstly show that the coolingrate has increased by a number of times, so that the residence time inthe furnace and the time for which the wafers are subject to elevatedtemperature are reduced considerably. Moreover, it is clear that thetemperature spread between the wafers 21 and 25 has been limited to aconsiderable extent, so that more constant process conditions can beensured.

Although the invention has been described above with reference to apreferred embodiment, it should be understood that it is possible fornumerous modifications to be made to this preferred embodiment. Forexample, the gas inlet openings and gas outlet openings may be arrangedat different positions over the height of the furnace, or may even openout at the top cover.

These and further variants, which will be obvious to the person skilledin the art after the above description has been read, lie within thescope of the appended claims.

What is claimed is:
 1. A method for cooling a core tube of a furnace,the core tube having an elongate boundary wall and being configured toaccommodate wafers for processing the wafers in a treatment atmosphere,the furnace having a cooling chamber defined between the elongateboundary wall and an outer casing of the furnace, wherein the outercasing includes a heating element and has first lateral,circumferentially spaced openings in proximity of a first end of theouter casing and second lateral, circumferentially spaced openings inproximity of a second end of the outer casing, comprising: supplying acooling gas through one of said first and second lateral,circumferentially spaced openings to a region of one end of said coolingchamber, wherein said cooling gas provides for a cooling atmosphere;guiding said cooling gas along said cooling chamber, wherein saidcooling gas has a uniform distribution of flow; discharging said coolinggas through one of said first and second lateral, circumferentiallyspaced openings from a region of an opposite end of the cooling chamber;periodically reversing a direction of flow of said cooling gas duringcooling; guiding said cooling gas along an essentially closed circuit sothat the cooling gas is preserved and so that the cooling atmosphere isseparated from the treatment atmosphere; and cooling said cooling gasthrough a liquid-gas heat exchanger.
 2. The method according to claim 1,wherein the direction of flow is reversed in intervals between 2 and 600seconds.
 3. The method according to claim 1, wherein the direction offlow is reversed in intervals between 5 and 60 seconds.
 4. The methodaccording to claim 1, wherein the direction of flow is reversed inintervals between 10 and 20 seconds.
 5. The method according to claim 1,wherein the furnace is cooled at a rate of approximately 50° C./min. 6.The method according to claim 1, wherein the gas is supplied anddischarged through said boundary wall in a metered fashion.
 7. Themethod according to claim 1, wherein said gas comprises nitrogen.
 8. Afurnace assembly, comprising: a furnace having an outer casing, a coretube located within the outer casing and a cooling chamber existingbetween the outer casing and the core tube, the core tube having anelongate boundary wall and being configured to accommodate wafers forprocessing in a treatment atmosphere, wherein the outer casing includesa heating element and has first lateral, cirucumferentially spacedopenings in proximity of a first end of the outer casing and secondlateral, circumferentially spaced openings in proximity of a second endof the outer casing; a gas line coupled to said first and secondlateral, circumferentially spaced openings to provide for communicationwith the cooling chamber at a first end and a second end of the furnaceto supply and to discharge a cooling gas that cools the boundary wall ofthe core tube and that has a uniform distribution of flow within thecooling chamber, the gas line including valves that comprise divertervalves to direct the cooling gas alternately in one direction along theboundary wall and then in an opposite direction along the boundary wall;and a liquid-gas heat exchanger in communication with the divertervalves forming a closed circuit for the cooling gas so that the coolinggas is preserved and so that the cooling atmosphere is separated fromthe treatment atmosphere.
 9. The furnace assembly according to claim 8,wherein a fan is arranged downstream of the heat exchanger.
 10. A methodfor cooling a core tube of a furnace, the core tube having an elongateboundary wall and being configured to accommodate wafers for processingthe wafers in a treatment atmosphere, the furnace having a coolingchamber defined between the elongate boundary wall and an outer casingof the furnace, wherein the outer casing includes a heating element andhas at least one first opening in proximity of a first end of the outercasing and at least one second opening in proximity of a second end ofthe outer casing, wherein the cooling chamber is closed at the first andsecond ends, comprising: supplying a cooling gas through one of saidfirst and second openings to a region of one end of said coolingchamber, wherein said cooling gas provides for a cooling atmosphere;guiding said cooling gas along said cooling chamber, wherein saidcooling gas has a uniform distribution of flow; discharging said coolinggas through one of said first and second openings from a region of anopposite end of the cooling chamber; periodically reversing a directionof flow of said cooling gas during cooling; guiding said cooling gasalong a closed circuit so that the cooling gas is preserved and so thatthe cooling atmosphere is separated from the treatment atmosphere; andcooling said cooling gas through a liquid-gas heat exchanger.
 11. Afurnace assembly, comprising: a furnace having an outer casing, a coretube located within the outer casing and a cooling chamber existingbetween the outer casing and the core tube, the core tube having anelongate boundary wall and being configured to accommodate wafers forprocessing in a treatment atmosphere, wherein the outer casing includesa heating element and has at least one opening in proximity of a firstend of the outer casing and at least one second opening in proximity ofa second end of the outer casing, wherein the cooling chamber is closedat the first and second ends; a gas line coupled to said first andsecond openings to provide for communication with the cooling chamber ata first end and a second end of the furnace to supply and to discharge acooling gas that cools the boundary wall of the core tube and that has auniform distribution of flow within the cooling chamber, the gas lineincluding valves that comprise diverter valves to direct the cooling gasalternately in one direction along the boundary wall and then in anopposite direction along the boundary wall; and a liquid-gas heatexchanger in communication with the diverter valves forming a closedcircuit for the cooling gas so that the cooling gas is preserved and sothat the cooling atmosphere is separated from the treatment atmosphere.